1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with spring retainer apparatus and method for facilitating loading of fuel rods into grid of a fuel assembly without incurring scratches on the exterior surfaces of the fuel rods.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a nuetron moderator. Specifically, in a boiling water reactor (BWR) the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces.
In a fuel assembly of this type the fuel rods in the central region of the bundle thereof may be undermoderated and overenriched. In order to remedy this condition by increasing the flow of moderator water through this region of the assembly, an elongated centrally-disposed water cross is frequently used in the assembly. The central water cross has a plurality of four radial panels which together form a cruciform water flow channel which divides the fuel assembly into four, separate elongated compartments, with the bundle of fuel rods being divided into mini-bundles disposed in the respective compartments. The water cross thus provides a centrally-disposed cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of, but separated from, adjacent fuel rods in the mini-bundles thereof. The fuel rods of each mini-bundle extend in laterally spaced apart relationship between an upper tie plate and a lower tie plate and connected together with the tie plates to comprise a separate fuel rod subassembly within each of the compartments of the channel. The water cross has approximately the same axial length as the fuel rod subassemblies, extending between the upper and lower tie plates thereof.
Additionally, a plurality of spacers or grids, for example six in number, are disposed at axially displaced positions along the fuel rods of each fuel rod subassembly to maintain the fuel rods in their laterally spaced relationships. The grids have interleaved straps which define a multiplicity of cells disposed in vertical alignment. The fuel rods are supported in an organized and transversely spaced array in the vertically aligned cells of the grids by springs and dimples on the straps which extend into the cells. Thus, the grids are introduced to maintain the desired fuel mini-bundle configuration and to prevent excessive fuel rod bow and flow induced vibrations, and thereby provide significant benefits from a structural standpoint.
In the common way of loading fuel rods into the fuel assembly grids containing the springs and dimples for holding the fuel rods in place, the fuel rods are ordinarily scraped by the springs and dimples as they enter the grid cells. Such scraping produces scratches which provide sites for accelerated corrision of the exterior surfaces of the fuel rods and debris for release into the coolant flow system of the reactor.
Devices are known for deflecting the springs of the fuel assembly grids for facilitating insertion of the fuel rods without marring the surfaces thereof. Representative of the prior art are the devices disclosed in U.S. Pat. Nos. 3,982,994 and 4,058,224 to Jabsen. While these prior art devices would appear to operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, they seem to embody shortcomings which make them less than an optimum solution to the problem of scratch-free loading of the fuel rods. The devices require openings in the straps of the grid through which their actuating parts must be inserted to position them for deflecting the grid springs. Such openings may tend to weaken the grid structure. Also, operation of such devices in deflecting the grid springs appears to be unduly complicated.
Consequently, a need exists for a fresh approach to insertion of fuel rods into the grids which will avoid or reduce the damage to the exterior surfaces of the fuel rods and at the same time be less complicated in operation.